Photolithography is the process of transferring geometric shapes on a photographic mask to the surface of a silicon wafer; it falls under the more broad category of microlithography. A photographic mask (or “photomask”), typically a glass plate with a patterned emulsion of metal film on one side, is used in photolithography to create integrated circuits. Chromium (chrome) is typically used to produce the pattern on the photomask.
The successful manufacture of advanced sub-micron sized semiconductor devices requires accuracy in production of the photomask, and in the photolithography processes used to pattern the wafer. Photolithography processes for semiconductor manufacturing frequently use image simulation for predicting the outcome of the manufacturing process. Simulation allows an evaluation of the quality of the product before spending time and money producing the actual product. The simulation takes as input either the electronic, geometrical design of the circuit to be produced, or the observed photomask image made from that design. The output is either a representation of the image as formed on the resist on the wafer, the so-called “aerial image,” or a representation of the result after the wafer has been exposed and developed.
The current standard procedure, as implemented in products such as VSS by Numerical Technologies, Inc. and ProLith by KLA-Tencor, is to use the Hopkins Method for modeling the electric fields that create the final image on the wafer. The Hopkins method is described in the following references, which are incorporated by reference: the Kirchauer Thesis; Professor Neureuther's work on UC Berkeley's “SPLAT” simulation program, available from the University of Berkeley; and A. K. Wong and A. R. Neureuther, Rigorous Three-Dimensional Time-Domain Finite-Difference Electromagnetic Simulation for Photolithographic Applications, IEEE Trans. Semicond. Manufact., 8(4):419-431, November 1995).
The Hopkins Method requires a large number of calculations, and therefore is quite slow. A faster technique for simulating an image would be highly desirable. Faster simulation is important anywhere simulation is used. In photomask defect detection and analysis it allows determination of defect severity on the resultant wafer at a rate similar to the speed of current mask inspection machines. This greatly reduces the number of false defects reported while increasing the available sensitivity of inspections. Reduced false defect reports decreases costs involved with 1) reviewing reported defects 2) repairing false defects 3) damage caused by repair, and 4) re-inspecting masks after repair. Increasing sensitivity allows using existing inspection machines for newer, smaller geometry chip designs.
Fast simulation is also important for chip design and photolithography process development. A faster simulation method would allow more iterations of a chip design to optimize feature placement and optical enhancement techniques. Recent products, such as KLA-Tencor's “DesignSmart” use large parallel computers to perform simulation. Fast simulation would allow this simulation to be accomplished by a single processor at much lower cost for both hardware and software.